The gasification of solid fuels, particularly of coal or brown coal, in a fixed bed in processes leaving a solid ash has been described, e.g., in Ullmanns Enzyklopadie der technischen Chemie, 4th edition (1977), Vol. 14, on pages 383 to 386. Details of that known gasifying process have also been published in U.S. Pat. Nos. 3,560,867 and 3,854,895. The gasification of fuel in a fixed bed processes including a tapping of liquid slag has been explained in British patent specifications Nos. 1,507,905, 1,506,671 and 1,512,677. In the process described in European Pat. No. 12 456 and the corresponding U.S. Pat. No. 4,295,864 the condensate which results from the cooling of the raw gas under superatmospheric pressure is pressure-relieved so that flashed-off vapor and a liquid phase are obtained. The flashed-off vapor is supplied to a combustion chamber and is burnt therein at temperatures above 800.degree. C. Part of the liquid phase must be removed from the process and must be aftertreated because otherwise the concentration of the impurities in the gas or in the condensate would increase to excessive levels. Considerable costs are incurred in the aftertreatment and purification of liquid.